The voltage in Japan is 100 Volt, which is different from North America (120V), Central Europe (230V) and most other regions of the world. Japanese electrical plugs have two, non-polarized pins, as shown above. They fit into North American outlets.
Japanese power outlets are identical to ungrounded (2-pin) North American outlets. While most Japanese outlets these days are polarized (one slot is slightly wider than the other), it is possible to encounter non-polarized outlets in some places.
Some North American equipment will work fine in Japan without an adapter and vice versa, however, certain equipment, especially equipment involving heating (e.g. hair dryers), may not work properly or even get damaged. If you intend to purchase electronic appliances in Japan for use outside of Japan, you are advised to look for equipment specifically made for oversea tourists.
The frequency of electric current is 50 Hertz in Eastern Japan (including Tokyo, Yokohama, Tohoku, Hokkaido) and 60 Hertz in Western Japan (including Nagoya, Osaka, Kyoto, Hiroshima, Shikoku, Kyushu); however, most equipment is not affected by this frequency difference. A possible exception are timing devices such as clocks.